Method of providing a printed circuit board with a cover coat

Abstract

 Method of providing a printed circuit board with a cover coat (19) on a selected areas, and in alignment with the circuit pattern thereon, which method comprises letting a photoresist (13) covering the areas not to be provided with the cover coat, react on blanked exposing the board to light, with an overlayer of another photosensitive material (17) covering the entire board, whereby the reacted portions (18) are rendered soluble, whereas the remaining portions defining the cover coat (19) are not.

Description

[0001] The present invention relates to providing printed circuit boards (PCB's) with a cover coat, and in particular, to a method for creating the cover coat pattern in a photoresist overlayer simultaneously with the pattern for pads, lines etc. in the copper foil of the board.

[0002] In the prior art, PCB's are manufactured in two basic steps. The first step is creating the pattern for the conducting material, e.g. copper, and the second step is to cover the PCB with a protective coating except on the contact pads.

[0003] In the first step a polyimide sheet substrate, e.g. of Kapton (Kapton is a registered trademark of DuPont Company), is provided with a thin foil or layer of a conductive material, such as copper. The copper is covered with a photoresist (e.g. AZ1375, which is a trademark of Shipley) and a photo mask containing the circuit pattern is placed on the resist. The photoresist is exposed to light through the mask, whereby the portions that were exposed to light, are susceptible to removal by a suitable developer, whereas the unexposed areas, defining the circuit pattern, remain. After exposure the mask is removed, and the soluble areas are washed away. The exposed copper is etched away leaving a circuit pattern. The photoresist is again exposed and developed, or removed by a suitable solvent thus finishing the printed circuit panel.

[0004] In the second step the normal procedure is to blanket coat a printed circuit panel, e.g. by silk screen printing, with a negative working photo-sensitive material (by negative working is understood that illuminated parts are polymerized, whereas remaining parts can be washed away; an example of such a material is cover coat 5500 which is a trademark of Lea Ronal). A photo mask, with a pattern for those areas to be covered, is aligned to the panel and exposed to light.

[0005] This alignment of the mask for the cover coat with those parts of the PCB that are not to be covered, i.e. the pads, is very difficult, especially when there is a high circuit density, and the operation is time consuming and thus very expensive.

[0006] Also known in prior art is the use of a special multi-density mask for the pattern generation. Such a mask is disclosed in IBM Technical Disclosure Bulletin Vol. 19, No. 12, May 1977, pages 4539 and 4540, and in Vol. 20, No. 3, August 1979, pages 964-5.

[0007] The described mask is such that opaque portions correspond to lands, partially transparent portions correspond to lines and fully transparent portions correspond to background substrate.

[0008] The present invention uses a modification of this mask, which will be described in detail later.

[0009] In view of the above mentioned disadvantages with prior art techniques, it is the object of the present invention to provide a method for cover coating printed circuit boards wherein the alignment problem is eliminated.

[0010] This is achieved by the invention as claimed in the appended claims in that the patterns for both the conductor lay-out and for the cover coat created simultaneously in one single exposure of the photo-resist through the mark.

[0011] An essential feature for being able to carry out the invention is an unexpected interaction between the photoresist and the cover coat upon exposure to light. It turns out that provided the layers of the respective materials are thin enough, there occurs a light induced reaction between the materials, rendering the exposed portions more soluble in a specific solvent, than the non-exposed portions.

[0012] One way of carrying out the invention is described in detail below with reference to the drawings, in which

Fig 1 is a schematic picture in top view of a mask for carrying out the invention, and

Fig 2.1 - 2.6 show the sequence of steps for making a PCB provided with a cover coat in accordance with the invention; the figures show cross-sections of a simplified circuit board during manufacturing.

[0013] The tri-level mask shown schematically in figure 1 has opaque portions 1 for creating contact pads, fully transparent portions 2 for background substrate and semi transparent (10-30% transmission) areas 3 for lines. Its function will be described below.

[0014] In Figs. 2.1 - 2.7, one way of carrying out the method according to the invention is shown as a sequence of steps.

[0015] Having prepared a suitably patterned mask Figure 1, a substrate comprising a base material 11 (eg: Kapton) covered over at least one surface (generally the whole surface is covered although this is not essential) with a thin foil of copper 12 and an overlayer of a photoresist 13 (eg Shipley AZ1375), is exposed to light through the mask 14 as shown is Figure 2.1. The photoresist is applied as a dispersion and dried before exposure.

[0016] Figure 2.2 shows the board after exposure and the selective wash away of those parts that were fully exposed, ie: those areas constituting the background substrate (Kapton), by using a mild developer (eg: AZ Developer).

[0017] The circuit is etched thus removing the exposed copper areas 15 to give the board as shown in Figure 2.3.

[0018] Figure 2.4 shows the board after a more aggressive developer (eg: microposit 351 developer Shipley) has be used to remove the resist - 16 from the areas exposed to the semi-transparent parts 3 of the mask 14, ie: the parts constituting the lines.

[0019] A cover coat in the form of a negative working photo sensistive material 17 such as Lea Ronal 5500 (an epoxy modified acrylic) is now blanket screened onto the panel as shown in Figure 2.5, dried and blanket exposed.

[0020] Exposure to light unexpectedly induces a modification of those parts - 18 of the cover coat material that interface with the photoresist (Figure 2.6) such that the modified material can be selectively dissolved along with the reaction products of the positive photoresist to leave a cover coated panel with coated areas 19 and bare pads 20 as shown in Figure 2.7.

[0021] The mechanism or the chemistry of this phenomenon is not fully understood, but is thought to be inhibition of the photo cross linking of the cover coat material by one or more components of the photo resist.

[0022] The cover coat is finally cured according to the manufacturers instructions.

[0023] Control of the time between applying and drying the cover coat allows some dissolution of the photoresist without excessive loss of definition by smearing (about 5 minutes is suitable).

[0024] The exposure time may be slightly increased to ensure complete scission of the positive photoresist under the cover coat.

[0025] The final development of the cover coat is carried out an follows:

1. 2 minute soak in aqueous alkaline solution with a pH of 12-13 containing sodium phosphates and silicates but no hydroxide .

2. Neutralize the alkali with 2-5% HCl.

3. Rinse in an organic solvent such as N-butyl-acetate or acetone.

[0026] Thus, by means of invention a simple and inexpensive method of providing a cover coat on a PCB is disclosed. lt should be evident to the skilled man that the invention can be varied within the scope of the appended claims.

Claims

1. Method of providing a printed circuit board with a cover coat on a selected area thereof
characterised by the steps of:

i) applying a photoresist material to an area of the board not to be provided with a covercoat;

ii) applying a photosensitive material to both the photoresist and the area of the board to be provided with covercoat;

iii) exposing the board to light to promote reaction of photoresist material with photosensitive material;

iv) removing the reaction product of photoresist material and photosensitive material;

and (v) curing the remaining photosensitive material to convert to covercoat.

2. Method according to claim 1 wherein the cover coat is provided in alignment with the circuit pattern on the board.

3. Method according to Claim 2, including the preliminary steps of:
simultaneously creating a latent image of the circuit pattern and of the cover coat respectively in a photoresist of the base material, by exposing the photoresist to light through a mask, and
individually developing the respective patterns, whereby the alignment of the cover coat on the circuit pattern is achieved.

4. Method according to Claim 3, wherein the mask is a multi-density mask defining the patterns for both the cover coat and the circuit pattern.

5. Method according to Claim 4, wherein the pattern of the mask is such that areas defining contact pads are opaque, areas defining circuit lines are semi-transparent, and areas defining background are fully transparent.

6. Method according to any of the preceding claims, wherein the photosensitive material is an epoxy modified acrylic and the photoresist is a formaldehyde novolak resin with a diazo ketone sensitizer.

7. Method according to any of the preceding claims, wherein the photosensitive material is allowed to dry for approximately 5 minutes after applying the same as an overlayer on the board, and before blanket exposing it to light.

8. Cover coat for a PCB, made by the method according to any of claims 1 - 5, characterised in that it is fully congruent with the circuit pattern except for the contact pads, which are exposed for the purpose of soldering components thereon.

9. Cover coat according to Claim 8, wherein the congruency is achieved by using one single photo mask and one single exposure for the manufacture of the entire PCB including conductive pattern and the cover coat.

Drawing